1. Field of the Invention
The present invention relates generally to sound image localizing apparatus used when sound signals are reproduced through speakers, and more specifically to a surround signal processing apparatus used when surround signals are reproduced in such way as to surround a listener or listeners. Here, the sound image implies a virtual sound source image from which a listener feels that a sound is reproduced. Further, the sound image can be localized at any desired position away from a speaker or speakers.
2. Description of the Prior Art
Conventionally, in the case where stereophonic sound is reproduced in such a way as to provide a sound field expanding behind a listener or to localize a sound image behind a listener, two front speakers are arranged in front of a listener for stereophonic sound reproduction and at least one or two rear speaker are additionally arranged behind the listener for surround sound reproduction; in other words, at least three speakers must be arranged at the minimum around a listener. Further, in the case where surround sound is reproduced on the basis of a one-system surround signal or a center channel is additionally required to be reproduced as with the case of the 3-1 system of high vision HDTV (High Definition TV), one or two additional center speakers must be arranged. Therefore, amplifiers and cables corresponding to the numbers of the reproduced channels are necessary.
In other words, as shown in FIG. 1A for instance, in the case of the surround sound reproduction, it has been necessary to arrange two front L(left)- and R(right)-channel speaker sets for stereophonic sound on front left and right sides of a listener LM, two rear SL(surround left)- and SR(surround right)-channel speaker sets for surround sound on rear left and right sides thereof, and further a C(center)-channel speaker at the front middle thereof, respectively.
In ordinary homes, however, since it is difficult to arrange the two rear speakers and the center speaker from the standpoint of space and cost, in practice as shown in FIG. 1B, only L- and R-channel speakers are installed on the front left and right sides of a listener LM. In this speaker arrangement, it has become impossible to obtain sufficient surround sound effect. In the case of the surround reproduction system using a monophonic surround signal in particular, although this system has such a feature that a sound field can be obtained on the rear side of a listener or the sound image can be shifted, it has been impossible to obtain such effects as described above without arranging the rear speakers.
Recently, however, a surround signal processing apparatus has been developed such that a stereophonic sound effect similar to the case where the rear speakers are arranged can be obtained on the basis of the sound reproduction through only the front left and right speakers.
In this surround signal processing apparatus, sound image localization signals obtained by transforming the rear channel signals are reproduced through two front speakers arranged at two predetermined positions in front of a listener, in addition to the original two (L- and R-) channel stereophonic signals. Alternatively, two pairs of speakers are arranged in front of a listener; only the original L- and R-channel signals are reproduced through one pair of the speakers; and the sound image localization signals are applied to the other pair of the speakers. On the basis of the sound image localization as described above, even if no rear speakers are arranged behind a listener in practice, it has become possible to reproduce surround sound in such a way that the listener can hear sound as if it came from the rear side of the listener.
In order to obtain the desired sound image localization signal by transforming the rear channel signal as described above, appropriate calculations are executed on the basis of the spatial transfer characteristics between a pair of speakers actually arranged and the left and right side ears of a listener and the spatial transfer characteristics between a speaker arranged only for measurement at one of the two predetermined rear speaker positions (at which sound images are required to be localized) and the left and right side ears of the listener. In other words, filter calculations are executed with the use of convolvers (convolution arithmetic processing circuits).
Here, a prior art surround signal processing apparatus using the sound image localization signals will be described hereinbelow with respect to its configuration and its principle.
FIG. 2 is a conceptual diagram showing the surround signal processing apparatus based upon the sound image localization technique. In the drawing, a surround signal processing apparatus 20 of the four channel type receives two channel stereophonic signals L and R, a center channel signal C for improving the localization of the middle position of the stereophonic sound, and a rear channel signal S for obtaining a surround stereophonic sound effect from the outside. Further, the processing apparatus 20 transforms the rear channel signal S and the center channel signal C into spatial localization signals for localizing sound signals at any desired sound image positions, respectively, in order to realize the surround reproduction in such a way that the reproduced sound can surround a listener LM.
In this sound image processing apparatus 20, it is possible to obtain a surround stereophonic sound effect by reproducing the stereophonic signals L and R and the transformed spatial localization signals through two speakers SP1 and SP2 arranged on the front left and right sides of a listener LM, without arranging two rear left and right speakers SP3 and SP4, a front middle speaker SP5, and a rear middle speaker SP6.
Further, FIG. 3 is an illustration for assistance in explaining a principle that a sound image can be localized at any given spatial position enclosing a listener LM by use of two stereophonic speakers SP1 and SP2. In FIG. 3, the transfer characteristics (the frequency response to an impulse) between the left side speaker SP1 and both left and right ears of a listener LM are denoted by h1L and h1R; and the transfer characteristics between the right side speaker SP2 and both left and right ears of the listener LM are denoted by h2L and h2R; and the transfer characteristics between a speaker assumed to be arranged at an intended localization position x and both left and right ears of the listener LM are denoted by pLx and pRx, respectively. Here, the respective transfer characteristics can be measured by arranging a speaker, a human head (or a dummy head) and two microphones (arranged at both the ear positions thereof). Further, the waveforms of the measured characteristics are processed appropriately.
Here, the case is taken into account where a sound source signal X required to be localized is passed through two signal transforming circuits 21A and 21B (whose transfer characteristics can be represented by cfLx and cfRx), respectively and further reproduced through two speakers SP1 and SP2, respectively. Then, the signals eL and eR received by the left and right ears of the listener LM can, using convolution operation, be expressed as: EQU eL=h1L*cfLx*X+h2L*cfRx*X (11a) EQU eR=h1R*cfLx*X+h2R*cfRx*X (11b)
On the other hand, when the source signal X is reproduced at the objective localized position, the signals dL and dR received by both left and right ears of the listener LM can be expressed as: EQU dL=pLx*X (12a) EQU dR=pRx*X (12b)
Now, if the signals reproduced by the speakers SP1 and SP2 and then received by both ears of the listener LM match the signals reproduced when the source signal X is reproduced at the objective localization position x, it is possible for the listener LM to recognize the sound image as if a speaker were arranged at the objective localization position x.
That is, the following formulae can be obtained by eliminating the source signal X on the basis of the conditions eL=dL and eR=dR and in accordance with the formulae (11a), (11b), (12a) and (12b): EQU h1L*cfLx+h2L*cfRx=pLx (13a) EQU h1R*cfLx+h2R*cfRx=pRx (13b)
Further, cfLx and cfRx can be obtained in accordance with the formulae (13a) and (13b) as: EQU cfLx=(h2R*pLx-h2L*pRx)*(1/H) (14a) EQU cfRx=(-h1R*pLx+h1L*pRx)*(1/H) (14b)
where EQU H=h1L*h2R-h2L*h1R (14c)
Accordingly, when the signal required to be localized is processed by the signal transforming circuits 21A and 21B (referred to as localization filters for a position x) provided with the transfer characteristics cfLx and cfRx calculated in accordance with the formulae (14a) to (14c) respectively, it is possible to localize a sound image at an objective localization position x.
In other words, a sound image can be localized at an objective position x by processing the surround signal through a pair of localization filters determined by setting a rear speaker arrangement position to a sound image localization position x, and further by reproducing the filtered sound source signal through the two front speakers SP1 and SP2, respectively. In practice, however, a surround signal processing apparatus has been so far constructed by combining a plurality of pairs of localization filters, as shown in FIGS. 4 or 5, respectively.
In more detail, FIG. 4 shows a prior art surround signal processing apparatus which can process the sound image localization in such a way that two rear channel signals SL and SR can be reproduced at two symmetrical rear left and right positions of a listener LM, on the basis of two channel stereophonic signals L and R, a single channel center signal C, and two rear channel surround (rear) signals SL and SR all outputted by a surround decoder SD.
In this processing apparatus, a pair of the localizing filters 21A and 21B is provided for each of the two rear channel signals SL and SR, and two sound image are localized at two positions of the two rear speakers SP3 and SP4, as shown in FIG. 2. That is, the signals obtained by addition of the signals L, R and C and the sound image localization processed signals are reproduced by a pair of the two front speakers SP1 and SP2, respectively. Therefore, in this processing apparatus, the sound image localization processing is made by use of 4 filters in total for two surround (rear) channel signals SL and SR.
Further, although not shown, there exists another processing apparatus such that the sound image localization processing can be made for the front channel signals L and R. In this processing apparatus, 8 filters are necessary in total, Furthermore, there exists another processing apparatus such that the sound image localization processing is executed for the front channel signals L and R and further for the center signal C. In this case, however, 10 filters in total are required.
On the other hand, FIG. 5 shows a prior art surround signal processing apparatus which can cope with a surround reproduction system using a monophonic (single system) rear surround signal. In this processing apparatus, a pair of localization filters (21A and 21B) for one channel are provided, and the surround signal S can be localized at the position of the speaker SP6, as shown in FIG. 2, by use of two filters in total.
In the above-mentioned prior art surround signal processing apparatus as shown in FIG. 4, however, since two pairs of the sound image localization processing filters (21A, 21B; 21A, 21B) are necessary for the rear stereophonic signals; that is, since 4 filters are necessary in total, the hardware scale inevitably increases, thus causing a problem in that this processing apparatus cannot be used for the ordinary home appliances such as television sets.
Further, in the case of the surround signal processing apparatus using the monophonic (one system) rear surround signal as shown in FIG. 5, since a sound image is localized at only one rear position, it is difficult to manifest the sound field behind the listener LM sufficiently and or manifest the sound image movement articulately, thus raising a problem in that a sufficient surround effect cannot be obtained.